It is well known that a remote unit's location within a communication system may be determined using an amplitude difference-based angle of arrival estimation (AD-AOA). Such a method of AD-AOA location using average energy values for the received prompt rays is described in commonly assigned U.S. Pat. No. 5,786,791, METHOD FOR DETERMINING AN ANGLE OF ARRIVAL OF A SIGNAL TRANSMITTED BY A REMOTE UNIT IN A COMMUNICATION SYSTEM, by Bruckert, the disclosure of which is hereby expressly incorporated herein by reference. Other methods are described in U.S. Pat. No. 4,636,796, RADIO DIRECTION FINDING SYSTEM, by Imazeki and U.S. Pat. No. 4,833,478, AUTOMATIC DIRECTION FINDER ANTENNA ARRAY, by Nossen, the disclosures of which are hereby expressly incorporated herein by reference. Briefly, these later methods determine the angle of arrival of a signal transmitted from a remote unit by analyzing the amplitude differences of signals received at multiple antennas at a base site.
In accordance with known methods of locating a remote unit transmitter in code division multiple access (CDMA) based communication systems, a high precision estimation of the prompt ray time-delay and amplitude is required. High precision estimation of the received signal amplitude at the main sector can be achieved by the location searcher with the main sector base station based on the method for prompt ray extraction with coherent integration, deconvolution and non-coherent integration procedures. However, the low received chip-to-noise ratio Ec/No at the adjacent sector (7 to 15 dB less than at the main sector) makes it impossible to use the location searcher at the adjacent sector to provide accurate AOA estimate.
Higher precision estimations may be obtained at the adjacent sector using the methods and apparatus described in the commonly assigned U.S. pat. application METHOD AND APPARATUS FOR ESTIMATING A CHANNEL METRIC by Henderson, et al. filed of even date herewith, the disclosure of which is hereby expressly incorporated by reference. Still, low signal-to-noise ratio (SNR) conditions can hamper the effectiveness of the location searcher in determining the AOA even given a higher precision estimation.
Therefore, there is a need for an improved method and apparatus for AD-AOA estimation in the presence of low signal-to-noise ratio (SNR) conditions at the adjacent sector location searcher.